Recently, a millimeter wave system for large, high-speed communication or vehicular radar has been developed. In such a millimeter wave system, a transmission line transition is used for coupling electromagnetic energy, for example, between a waveguide tube and a planar line (e.g., a microstrip line) formed on a dielectric substrate.
As shown in FIGS. 9A and 9B, a conventional transmission line transition, for example, disclosed in JP-H11-261312A includes a dielectric substrate P1 (FIG. 9B) and a waveguide tube consisting of first and second waveguide members P2, P3 that are fixed to each other through the dielectric substrate P1. A microstrip line P4 and a ground plane P6 (FIG. 9B) are disposed on first and second surfaces of the dielectric substrate P1, respectively. The tip portion of the microstrip line P4 is positioned inside the waveguide tube and acts as an antenna P5 for exciting the waveguide tube.
The millimeter wave system consists of very small components. Therefore, manufacturing variations may be caused when the components are formed and assembled. The manufacturing variations cause characteristic variations between the manufactured systems.
For example, in the case of the transition shown in FIGS. 9A and 9B, it is difficult to accurately form the first waveguide member P2 and to accurately fix the first waveguide member P2 to the dielectric substrate P1. Therefore, is not suited for mass-production.
A distance between the tip of the antenna P5 and the ground plane P6 determine characteristics of the transition. As shown in FIG. 9B, the second waveguide member P3 is fixed to the ground plane P6. Therefore, if the second waveguide member P3 is fixed to an incorrect position on the ground plane P6, the transition has characteristics different from desired characteristics.
To reduce the manufacturing variations, the components of the transition need to be highly accurately formed and assembled. As a result, manufacturing time and cost of the transition is increased.